Components that are exposed to hot corrosive gases, like for example gas turbine components which are exposed to the hot and corrosive combustion gas, are typically made of superalloys which offer high strength at elevated temperatures. Such superalloys are, in particular, nickel based super alloys, iron based superalloys and cobalt based superalloys.
Although superalloys show high strength at temperatures up to 1,200° C. and more they need to be protected from oxidation and/or corrosion. This protection is typically done by alumina forming coatings such as so called MCrAlY-overlay coatings, in which M stands for iron (Fe), cobalt (Co) or nickel (Ni) and Y represents yttrium (Y), hafnium (Hf) or a rare earth element. During the preparation of the overlay coating the aluminium is oxidized and forms a tightly adherent scale of alumina which protects the components from oxidation and corrosion. Such coatings, for example, are known from EP 0 486 489 B1, EP 0 786 017 B1 or EP 1 306 456 A1.
Another method of forming corrosion and/or oxidation resistant scale of alumina is the so called slurry aluminising. In a slurry aluminising process an aluminium containing slurry is coated onto the surface of the component. Then, the component is subjected to a heat treatment which leads to melting of the aluminium. The melted aluminium then reacts with the material of the component and forms an aluminide coating with a scale of alumina protecting the component from oxidation and/or corrosion. In addition to aluminium, the slurry can also contain silicon leading to an aluminide-silicide coating which shows better oxidation resistance than the aluminide coating alone. Forming aluminide coatings and aluminide-silicide coatings by a slurry process is, for example, disclosed in U.S. Pat. No. 5,547,770, U.S. Pat. No. 6,080,246, EP 1 820 875 A2, US 2007/0298269 A1 and U.S. Pat. No. 6,126,758.